Screen printing is a printing technique that typically uses a screen made of woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink as a sharp-edged image onto a substrate. A roller or squeegee is moved across the screen with ink-blocking stencil, forcing or pumping ink past the threads of the woven mesh in the open areas. Graphic screen-printing is widely used today to create many mass or large batch produced graphics, such as posters or display stands. Full colour prints can be created by printing in CMYK (cyan, magenta, yellow and black (‘key’)). Screen-printing is often preferred over other processes such as dye sublimation or inkjet printing because of its low cost and ability to print on many types of media.
A significant characteristic of screen printing is that a greater thickness of the ink can be applied to the substrate than is possible with other printing techniques. Screen-printing is therefore also preferred when ink deposits with the thickness from around 5 to 20 micrometer or greater are required which cannot (easily) be achieved with other printing techniques. This makes screen-printing useful for printing solar cells, electronics etc. (The definition of ink in this application not only includes solvent and water-based [pigmented] ink formulations but also includes [colourless] varnishes, adhesives, metallic ink, conductive ink, and the like.)
Generally, a screen is made of a piece of porous, finely woven fabric called mesh stretched over a frame of e.g. aluminium or wood. Currently most meshes are made of man-made materials such as steel. As mentioned above, areas of the screen are blocked off with a non-permeable material to form the stencil, which is a negative of the image to be printed; that is, the open spaces are areas where the ink will appear.
In the process of printing, the screen having a stencil facing the substrate is placed atop a substrate such as paper or fabric. In conventional flatbed screen printing, ink is placed on top of the screen, and a fill bar (also known as a floodbar) is used to fill the mesh openings with ink. The operator begins with the fill bar at the rear of the screen and behind a reservoir of ink. The operator lifts the screen to prevent contact with the substrate and then using a slight amount of downward force pulls the fill bar to the front of the screen. This effectively fills the mesh openings with ink and moves the ink reservoir to the front of the screen. The operator then uses a squeegee (rubber blade) to move the mesh down to the substrate and pushes the squeegee to the rear of the screen. The ink that is in the mesh opening is pumped or squeezed by capillary action to the substrate in a controlled and prescribed amount. The theoretical wet ink deposit is estimated to be equal to the thickness of the mesh and or stencil, as will be discussed hereinafter. As the squeegee moves toward the rear of the screen the tension of the mesh pulls the mesh up away from the substrate (called snap-off) leaving the ink upon the substrate surface. In rotary screen printing, the ink is typically forced from the inside of the cylindrical screen. Nowadays, this process is automated by machines.
There are three types of screen-printing presses. The ‘flat-bed’ (probably the most widely used), ‘cylinder’, and ‘rotary’. Flat-bed and cylinder presses are similar in that both use a flat screen and a three step reciprocating process to perform the printing operation. The screen is first moved into position over the substrate, the squeegee is then pressed against the mesh and drawn over the image area, and then the screen is lifted away from the substrate to complete the process. With a flat-bed press the substrate to be printed is typically positioned on a horizontal print bed that is parallel to the screen. With a cylinder press the substrate is mounted on a cylinder. Stability of the image can be a problem due to the movement of the metal threads of a woven screen. On the other hand, rotary screen presses are designed for continuous, high speed web printing. The screens used on rotary screen presses are for instance seamless thin metal cylinders. The open-ended cylinders are capped at both ends and fitted into blocks at the side of the press. During printing, ink is pumped into one end of the cylinder so that a fresh supply is constantly maintained. The squeegee, for instance, is a free floating steel bar inside the cylinder and squeegee pressure is maintained and adjusted for example by magnets mounted under the press bed. Rotary screen presses are most often used for printing textiles, wallpaper, and other products requiring unbroken continuous patterns.
Screen-printing is more versatile than traditional printing techniques. The surface does not have to be printed under pressure, unlike etching or lithography, and it does not have to be planar. Screen-printing inks can be used to work with a variety of substrates, such as textiles, ceramics, wood, paper, glass, metal, and plastic. As a result, screen-printing is used in many different industries.
One of the interesting areas for screen printing is in inks that can be used to create raised images, smooth shining solid areas, or fine line patterns that appeal to both the tactile and visual senses. An improvement in respect of the quality of such printings would be rather desirable.
In particular for quality prints as indeed is the case for Braille printing, the process requires an extremely uniform relatively thick coating of ink without ghosting or streaks. It would therefore be very interesting to be able to improve the uniform deposition of increased amounts of ink on substrates, especially for finer details. This would be of interest in flatbed and cylinder screen printing and rotary printing alike.
In addition to screens made on the basis of a woven mesh based on metal threads, such as U.S. Pat. No. 3,759,799, screens have been developed out of a solid metal sheet with a grid of holes. In U.S. Pat. No. 4,383,896 or U.S. Pat. No. 4,496,434 for instance, and in subsequent patents by the current applicant, a metal screen is described comprising ribs and apertures. This screen is prepared by a process comprising of electrolytically forming a metal screen by forming in a first electrolytic bath a screen skeleton upon a matrix provided with a separating agent, stripping the formed screen skeleton from the matrix and subjecting the screen skeleton to an electrolysis in a second electrolytic bath in order to deposit metal onto said skeleton. This technique has been used to prepare metal screens for screen printing with various mesh sizes (e.g. from 75 to over 350), thicknesses (from about 50 to more than 300 micrometer), and hole diameters (from 25 micrometer and greater) and thus various amounts of open area (from about 10 to about 55%), wet ink deposits (from about 5 to more than 350 micrometer thick) and resolution (from about 90 to 350 micrometer). Indeed, these screens outperform woven screens in terms of lifetime, sturdiness and stability, resistance to wrinkling with virtually no breakages or damage during press set-up or printing. Still, it would be of interest to improve such non-woven screens in respect of greater ink deposition and sharper images. Accordingly, this is one of the aims of the current invention.
Moreover, as mentioned before, screen printing is ideal for preparing wafer-based solar PV cells. The preparation of such cells comprises printing ‘fingers’ and buses of silver on the front; and buses of silver printed on the back. The buses and fingers are required to transport the electrical charge. On the other hand, the buses and fingers need to take as little surface of the solar PV cells as possible, and thus tend to be relatively thick. Screen printing is ideal as one of the parameters that can be varied greatly and can be controlled fittingly is the thickness of the print.
Solar wafers are becoming thinner and larger, so careful printing is required to maintain a low breakage rate. On the other hand, high throughput at the printing stage improves the throughput of the whole cell production line.
Rotary screen-printing is typically a roll-to-roll technology, which enables continuous high volume and high speed production. Further benefits include reduced ink and chemical waste, higher ink deposits, great production flexibility (various repeat sizes and web widths), with excellent quality, repeatable results and reliable performance.
The application of electronics on common substrates such as paper, film and textile using rotary screen-printing is relatively new. Rotary screen technology enables low cost production of printed electronics, such as radio-frequency identification tags (RFID tags).
For instance, Stork Prints has designed various rotary screen printing lines especially for printed electronics applications. Their machine parts are specifically developed for high accuracy printing on (heat) sensitive substrates. For instance, the design of the PD-RSI 600/900 rotary screen printing line (Stork Prints brochure 101510907) enables the production of an entire RFID tag in one run, at a speed of over 50,000 units per hour.
However, the demands being placed on screen-printing forms for graphics and especially printed electronics applications are increasing as components become smaller and the demand for high productivity fabrication processes intensifies. Printed lines widths of less than 80 micrometer combined with high ink transfer, durable print forms and excellent repeatability are becoming increasingly common. Despite the many benefits of screen-printing with non-woven screens, and in particular with rotary screen-printing; for very high resolution printing flatbed woven screen material still provides superior resolution and sharpness. Indeed, even the use of screens with a (very) high open area, and with smaller bridges making up the mesh, prints with printed lines widths less than 100 micrometer made with rotary screen-printing can be less sharp and result in less ink-transfer than prints made using the best flat-bed woven metal screen. Thus, it would be of great interest to find an improved screen that has all the strength and durability properties of the non-woven screens such as developed by Stork Prints, but with improved sharpness and ink-transfer capabilities for the preparation of highs resolution prints. Moreover, it would be of great interest to find a non-woven screen that can be applied in rotary screen printing, where woven metal screens cannot be used.
Interestingly, both problems of improved ink deposition and sharper printing have been solved through the application of a new type of screen.